This invention relates to a laser colorant removal imaging process, and more particularly to a process for applying a colorless, abrasion-resistant overcoat on an element obtained by such a process.
In recent years, thermal transfer systems have been developed to obtain prints from pictures, which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Another way to thermally obtain a print using the electronic signals described above is to use a laser instead of a thermal printing head. In such a system, the donor sheet includes a material which strongly absorbs at the wavelength of the laser. When the donor is irradiated, this absorbing material converts light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to its vaporization temperature for transfer to the receiver. The absorbing material may be present in a layer beneath the dye and/or it may be admixed with the dye. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. Further details of this process are found in GB 2,083,726A, the disclosure of which is hereby incorporated by reference.
In one ablative mode of imaging by the action of a laser beam, an element with a dye layer composition comprising an image dye, an infrared-absorbing material, and a binder coated onto a substrate is imaged from the dye side. The energy provided by the laser drives off the image dye and binder at the spot where the laser beam hits the element. In ablative imaging, the laser radiation causes rapid local changes in the imaging layer thereby causing the material to be ejected from the layer. This is distinguishable from other material transfer techniques in that some sort of chemical change (e.g., bond-breaking), rather than a completely physical change (e.g., melting, evaporation or sublimation) causes an almost complete transfer of the image dye rather than a partial transfer. Usefulness of such an ablative element is largely determined by the efficiency at which the imaging dye can be removed on laser exposure. The transmission Dmin value is a quantitative measure of dye clean-out: the lower its value at the recording spot, the more complete is the attained dye removal.
One way to improve abrasion resistance of such an element is to use lamination. Lamination involves placing a durable and/or adhesive protective layer coated on a suitable support to the image which is to be protected. The support of the protective coating may remain permanently adhered or it may subsequently be peeled off leaving only the protective layer adhered to the image. The protective layers described in the prior art are continuous polymeric coatings which have the disadvantage that air pockets may be trapped during the laminating step leading to image defects.
Another commonly used method for protecting images from surface damage is to apply a liquid overcoat. This method may avoid the problem of air trapping, but has many other problems, such as handling of liquids which may be messy or difficult to dry and cure, potential dissolution of the image, and the use of environmentally undesirable solvents.
In U.S. Pat. No. 5,429,909, a polymeric protective overcoat is applied to the surface of a laser ablative imaging element prior to the laser-writing process. However, there is a problem with this method in that a relief pattern of image dye and overcoat protrudes from the surface of the support and is sensitive to scratch and abrasion.
U.S. Pat. No. 5,847,738 discloses the electrostatic application and fusing of clear toner materials for protection of such an image. There is a problem with this method, however, in that a rather complex toner development system is required along with fusing hardware.
It is an object of this invention to provide an overcoat layer on an ablative recording element which is applied after imaging, so that the overcoat layer covers the entire surface of the ablative element, thus eliminating the protruding relief image problem. It is another object of this invention of provide an overcoat layer on an ablative recording element which does not trap air in the pockets of the relief image during the lamination step.
These and other objects are achieved in accordance with the invention which relates to a process of forming a single color, ablation image having improved abrasion resistance comprising:
a) imagewise-heating, by means of a laser, an ablative recording element comprising a support having thereon an image layer comprising a colorant dispersed in a polymeric binder, which causes the image layer to ablate imagewise, the image layer having a near infrared-absorbing material associated therewith to absorb at a given wavelength of the laser used to expose the element, the colorant absorbing in the region of from about 300 to about 700 nm; and
b) laminating a coating comprising polymeric particles dispersed in a binder to the surface of the ablative image under heat and pressure so that the particles will fuse into a continuous overcoat layer.
By use of the invention, an overcoat layer is obtained which covers the entire surface of the ablative element, thus eliminating the protruding relief image problem. In addition, the overcoat layer does not trap air in the pockets of the relief image during the lamination step.